Explain how the structure of a DNA nucleotide is related to its role as a monomer in the formation of a polynucleotide strand.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A DNA nucleotide consists of a deoxyribose sugar, a phosphate group and one of four nitrogenous bases.
Adjacent nucleotides are linked by covalent phosphodiester bonds between the phosphate of one and the 3‑hydroxyl of the next deoxyribose.
These repeated sugar–phosphate linkages form the backbone of the polynucleotide strand.
The nitrogenous base projects inward from the backbone and is available for complementary base‑pairing, enabling the strand to store and transmit genetic information.
Adjacent nucleotides are linked by covalent phosphodiester bonds between the phosphate of one and the 3‑hydroxyl of the next deoxyribose.
These repeated sugar–phosphate linkages form the backbone of the polynucleotide strand.
The nitrogenous base projects inward from the backbone and is available for complementary base‑pairing, enabling the strand to store and transmit genetic information.
Examiner tips
- Use the word "phosphodiester" to show knowledge of the bond type.
- Show the directionality of the backbone (5'‑3').
- Mention that bases are available for base‑pairing.
Common mistakes
- Forgetting to mention the phosphodiester bond.
- Describing the backbone as made of bases instead of sugar‑phosphate.
- Not stating that bases are available for complementary pairing.
Mark scheme (4 marks)
- A DNA nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases.
- Adjacent nucleotides are joined by covalent (phosphodiester) bonds formed between the phosphate group of one nucleotide and the deoxyribose sugar of the next.
- This repeated sugar–phosphate linkage forms the sugar–phosphate backbone of the polynucleotide strand.
- The nitrogenous base of each nucleotide projects inward (away from the backbone) and is available for complementary base pairing, which allows the strand to carry and transmit genetic information.
Key terms in this question
nucleotide · polynucleotide
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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